The relative weighting of individual channels in an array based imaging or sensing system can significantly change the sensitivity and resolution of a beamformer. FIG. 1 schematically illustrates a prior art Delay and Sum (DAS) beamformer 1000 in which dynamic focusing is performed by the application of time delays 1002 to the channels of signals 1004 received from the transducer array 2. The dynamically focused channels of signals 1006 are then weighted by scalar apodization weights 1008 (“W”, “X”, and “Y”, respectively in FIG. 1) and the weighted channels of signals 1010 are then summed by a summer 1012. In medical ultrasound imaging, the summed channel output is envelope detected to form an “A Line,” which is combined with additional A Lines to form a B-mode image, as known in the art.
Apodization is also applied to transmit beamformers in order to alter the beam shape, lower sidelobe levels, and improve depth of field. Depth of field refers to the range over which the transmitted beam is reasonably focused. Apodization on transmit can be implemented in the most straightforward manner by simply changing the amplitude of waveforms transmitted by different array elements. Given that transmit circuits rarely have the ability to arbitrarily change the amplitude of the transmit waveform, it is often easier to implement transmit apodization by using pulse width modulation to alter the effective power transmitted by each element. Conventional apodization functions, like the rectangular, Hamming, or Nuttall window, (e.g., see Nuttall, “Some Windows with Very Good Sidelobe Behavior”, IEE Trans. Acoust., Speech, and Signal Process., col. 29, no. 1, pp. 84-91, 1981, which is hereby incorporated herein, in its entirety, by reference thereto) typically offer a tradeoff between the width of the main lobe of the system spatial impulse response and the sidelobe levels (i.e., heights of the sidelobes). It is also notable that the selection of these conventional apodization functions is based upon the assumption that the imaging or sensing system is operating at a range from the array that occurs in the far-field of the array. This is almost never true for medical ultrasound imaging and is dubious for many other applications. Thus these conventional windows, though widely used, are known to be an imperfect solution. Furthermore, these windows are further limited because they are derived for a single operating frequency and modem array based imaging and communications systems almost entirely operate in a broad band mode.
Because receive channel weighting changes the shape of the overall system point spread function (PSF), the particulars of the applied apodization function greatly affect the contrast and resolution of the final output image.
Russell, U.S. Pat. No. 4,841,492 discloses the use of resistors to attempt to achieve a selected percent Gaussian apodization of a focused ultrasound wavefront when transmitted from a linear or phased array. In this case apodization is applied to the transmit beamformer, rather than the receive beamformer. Russell uses a string of resistors, one resistor positioned between the drivers of each element in the transducer array.
Lee et al. in “A hardware efficient beamformer for small ultrasound scanners, 2005 IEEE Ultrasonics Symposium, describes a digital receive beamformer that uses fractional delay (FD) filters to generate delayed samples in order to reduce the complexity of existing interpolation beamformers. Generally, FD filters are well known in the art and have been employed successfully to enable the application of focusing delays that are substantially smaller than that signal sampling rate. While FD filters do improve image quality, their design (i.e. determination of the proper degree of sub-sample delay) must be determined empirically. Furthermore, FD filters can only achieve image quality that is equivalent to the use of continuously varying focal delays (i.e. no delay quantization). The application of such continuously varying delays will not achieve the optimal image quality (contrast and resolution) possible for a given system.
There is a continuing need in the art for systems and methods for optimizing the directionality, sensitivity, contrast, and resolution of sensing, imaging, and communications systems that use arrays of sensors (or sources). This need is particularly acute in near-field and broadband signal applications. In the art of medical ultrasound imaging there exists a need for improved receive beamformers and methods for designing such beamformers to improve image contrast, resolution, and robustness to noise and tissue inhomogeneities.